Conventional methods for the detection and characterization of cell surface antigens involve surface radiolabeling with .sup.125 I(Morrison, 1980, Methods Enzymol. 70:214-220), whereas analysis of the total cellular protein pool is usually achieved by metabolic radiolabeling with [.sup.35 S]methionine and [.sup.35 S]cysteine(Coligan et al., 1983, Methods Enzymol. 91:413-434). Biotin is a powerful, nonradioactive reagent for labeling proteins and has been used to successfully label cell surface proteins on intact (Meier et al., 1992, Anal. Biochem. 204:220-226; Nesbitt and Horton, 1992, Anal. Biochem. 206:267-272) and permeabilized (Altin and Paglet, 1995, Anal. Biochem. 224:382-389; Altin et al., 1994, Immunol. 83:420-429) cells, but, being unsuitable for metabolic labeling, has not been considered an adequate alternative to radioisotopes for labeling intracellular proteins.